Human factor VIII:C (FVIII) is the coagulation factor deficient in the X-chromosome-linked bleeding disorder hemophilia A, a major source of hemorrhagic morbidity and mortality in affected males. Traditionally, hemophiliacs were treated with transfusions of whole blood. More recently, treatment has been with preparations of FVIII concentrates derived from human plasma. However, the use of plasma-derived product exposes hemophiliac patients to the possible risk of virus-transmissible diseases such as hepatitis and AIDS. Costly purification schemes to reduce this risk increases treatment costs. With increases in costs and limited availability of plasma-derived FVIII, patients are treated episodically on a demand basis rather than prophylactically. Recombinantly produced FVIII has substantial advantages over plasma-derived FVIII in terms of purity and safety, as well as increased availability and accordingly, much research effort has been directed towards the development of recombinantly produced FVIII.
Due to the labile nature of FVIII, especially following its activation, large and repeated doses of protein whether plasma or recombinantly-derived, must be administered to achieve a therapeutic benefit. However, the amount of FVIII protein the patient is exposed to has been correlated with the development of antibodies which inhibit its activity. In light of this known immunogenicity, one of the goals in developing new recombinant forms of FVIII for use as a therapeutic agent is the development of products that reduce or eliminate such an immune response.
FVIII functions in the intrinsic pathway of blood coagulation as a cofactor to accelerate the activation of factor X by factor IXa, a reaction that occurs on a negatively charged phospholipid surface in the presence of calcium ions. FVIII is synthesized as a 2351 amino acid single-chain polypeptide having the domain structure A1-A2-B-A3-C1-C2. Wehar, G. A. et al., Nature 312:337-342 (1984) and Toole, J. J. et al., Nature 312:342-347 (1984). The domain structure of FVIII is identical to that of the homologous coagulation factor, factor V (FV). Kane, W. H. et al., PNAS (USA) 83:6800-6804 (1986) and Jenny, R. J. et al., PNAS (USA) 84:4846-4850 (1987). The FVIII A-domains are 330 amino acids and have 40% amino acid identity with each other and to the A-domain of FV and the plasma copper-binding protein ceruloplasmin. Takahashi, N. et al., PNAS (USA) 81:390-394 (1984). Each C-domain is 150 amino acids and exhibits 40% identity to the C-domains of FV, and to proteins that bind glycoconjugates and negatively charged phospholipids. Stubbs, J. D. et al., PNAS (USA) 87:8417-8421 (1990). The FVIII B-domain is encoded by a single exon and exhibits little homology to any known protein including FV B-domain. Gitschier, J. et al., Nature 312:326-330 (1984) and Cripe, L. D. et al., Biochemistry 31:3777-3785 (1992).
FVIII is secreted into plasma as a heterodimer of a heavy chain (domains A1-A2-B) and a light chain (domains A3-C1-C2) associated through a noncovalent divalent metal ion linkage between the A1- and A3-domains. In plasma, FVIII is stabilized by binding to von Willebrand factor (vWF). More specifically, the FVIII light chain is bound by noncovalent interactions to a primary binding site in the amino terminus of von Willebrand factor. FVIII binds to phospholipid (PL) membranes, to vWF and to factor IXa via motifs localized to the C2 domain. Binding of FVIII to von Willebrand factor is mediated by epitopes within the terminal C2 domain as well as a contribution from the N-terminal acidic region (AR). PL binding is mediated by the terminal C2 domain. Previous work has demonstrated that the PL and vWF binding sites are overlapping and are competitive. Foster, P. A. et al., Blood, 75(10):1999-2004 (1990); Saenko, E. L. et al., J. Biol. Chem., 269(15):11601-5 (1994); and Healey, J. F. et al., Blood, 92(10):3701-9 (1998).
It has also been shown that PL binding and vWF binding are mediated by two pairs of hydrophobic residues, each displayed at the tips of β-hairpin turns. Pratt, K. P. et al., Nature, 402(6760):439-42 (1999) and Barrow, R. T. et al., Blood, 97(1):169-74 (2001). The homologous hydrophobic residues in the C2 domain of factor V also contribute to PL binding. It is believed that the solvent-exposed hydrophobic residues of the FVIII C2 make specific contacts with both PL and factor IXa, rather than merely providing hydrophobic surface area.
Upon proteolytic activation by thrombin, FVIII is activated to a heterotrimer of 2 heavy chain fragments (A1, a 50 kDa fragment, and A2, a 43 kDa fragment) and the light chain (A3-C1-C2, a 73 kDa chain). The active form of FVIII (FVIIIa), also known as thrombin-activated factor VIII, thus consists of an A1-subunit associated through the divalent metal ion linkage to a thrombin-cleaved A3-C1-C2 light chain and a free A2 subunit associated with the A1 domain through an ion association (see FIG. 1A). Eaton, D. et al., Biochemistry 25: 505 (1986); Lollar, P. et al., J. Biol. Chem. 266: 12481 (1991); and Fay, P. J. et al., J. Biol. Chem. 266: 8957 (1991).
This FVIIIa heterotrimer is unstable and subject to rapid inactivation through dissociation of the A2 subunit under physiological conditions. A homology model (Pemberton, S. et al., Blood 89(7):2413-21 (1997)) of the triplicated A domains of FVIII predicts a pseudo-threefold axis at the tightly packed hydrophobic core with several interdomain interactions. These lie at the interface of A1-A2, A2-A3 and A1-A3. Hemophilia A mutations (R531H, A284E, S289L) within the predicted A1-A2 interface disrupt potential intersubunit hydrogen bonds and have the molecular phenotype of increased rate of inactivation of FVIIIa due to increased rate of A2 subunit dissociation. Patients with these mutations exhibit a clinical phenotype here the FVIII activity by one-stage (1-st) assay is at least two-fold higher than by two-stage(2-st) assay.
FVa and FVIIIa are inactivated by Activated protein C (APC) in the presence of phospholipid and CaCl2 and APC-resistance has been considered to be one of the major causes of hereditary thrombophilia. Dahlbäck, B. et al., PNAS (USA) 90: 1004 (1993). The molecular basis for the APC-resistance was attributed to resistance to PC cleavage and inactivation. Dahlbäck, B. et al., PNAS (USA) 91: 1396 (1994). Previous studies on the APC inactivation of FVIII noted the generation of a 45 kDa fragment (Fulcher, C. A. et al., Blood 63: 486 (1984)) derived from the amino-terminus of the heavy chain and was proposed to result from cleavage at Arg336. Eaton, D. et al., Biochemistry 25: 505 (1986). While the light chain of FVIII is not cleaved by APC, multiple polypeptides, representing intermediate and terminal digest fragments derived from the heavy chain, have been observed. Walker, F. J. et al., Arch. Bioch. Biophys. 252: 322 (1987). These fragments result from cleavage site locations at Arg336, the unction of the A1 and A2 domain, at Arg562, bisecting the A2 domain, and a site at the A2-B junction, likely at Arg740. Fay, P. J. et al., J. Biol. Chem. 266: 20139 (1991). APC cleavage of FVIII at residue 336 generates a 45 kDa fragment from the amino-terminus of the A1-domain and cleavage at residues 562 and 740 generates a 25 kDa fragment from the carboxy-terminus of the A2-domain (see FIG. 1A).
Previous transfection studies demonstrated that FVIII is 10-fold less efficiently secreted than FV. The inefficient secretion of FVIII correlates with binding to the protein chaperonin identified as the immunoglobulin binding protein (BiP), also known as the glucose-regulated protein of 78 kDa (GRP78) (Munro, S. et al., Cell 46:291-300 (1986)) within the lumen of the ER (Dorner, A. J. et al., EMBO J. 4:1563-1571 (1992)). BiP is a member of the heat-shock protein family that exhibits a peptide-dependent ATPase activity. Flynn, G. C. et al., Science 245:385-390 (1989). BiP expression is induced by the presence of unfolded protein or unassembled protein subunits within the ER. Lee, A. S., Curr. Opin. Cell Biol. 4:267-273 (1992) and Kozutsumi, Y. et al., Nature 332:462-464 (1988). It has been shown that high level FVIII expression induces BiP transcription. Dorner, A. J. et al., J. Biol. Chem. 264:20602-20607 (1989). In addition, FVIII release from BiP and transport out of the ER requires high levels of intracellular ATP. Dorner, A. J. et al., PNAS (USA) 87:7429-7432 (1990). In contrast, it has been found that FV does not associate with BiP and does not require high levels of ATP for secretion. Pittman, D. D. et al., J. Biol. Chem. 269: 17329-17337 (1994). Deletion of the FVIII-B-domain yielded a protein that bound BiP to a lesser degree and as more efficiently secreted. Dorner, A. J. et al., J. Cell Biol. 105:2665-2674 (1987). To evaluate whether the FVIII B-domain was responsible for BiP interaction, FV and FVIII chimeric cDNA molecules were constructed in which the B-domain sequences were exchanged. Pittman, D. D. et al., Blood 84:4214-4225 (1994). A FVIII hybrid harboring the B-domain of FV was expressed and secreted as a functional molecule, although the secretion efficiency of the hybrid was poor, similar to wild-type FVIII. Pittman, D. D. et al., Blood 84:4214-4225 (1994). This indicated that the difference in secretion efficiency between FV and FVIII was not directly attributable to specific sequences within the FVIII B-domain, the most divergent region between these homologous coagulation factors.
To determine whether specific amino acid sequences within FVIII A-domain inhibit secretion, chimeric proteins containing the A1- and A2-domains of FVIII or FV were studied. The chimeric protein containing the A1- and A2-domains of FV was secreted with a similar efficiency as wild-type FV. The complementary chimera having the A1- and A2-domains of FVIII was secreted with low efficiency similar to wild-type FVIII. These results suggested that sequences within the A1- and A2-domains were responsible for the low secretion efficiency of FVIII. An A1-domain-deleted FVIII molecule was constructed and secretion was increased approximately 10-fold compared to wild-type FVIII A2-domain-deleted FVIII. Expression of the FVIII A1-domain alone did not yield secreted protein, whereas expression of the FVIII A2-domain alone or the FV A1-domain or A2-domain alone directed synthesis of secreted protein. Secretion of a hybrid in which the carboxyl-terminal 110 amino acids of the A1-domain were replaced by homologous sequences from the FV A1-domain (226-336 hybrid FVIII) was also increased 10-fold compared to wild-type FVIII, however, the secreted protein was not functional, i.e. did not display procoagulant activity, and the heavy and light chains were not associated. Marquette, K. A. et al., J. Biol. Chem. 270:10297-10303 (1995). It would thus be desirable to provide a functional recombinant FVIII protein having increased secretion as compared to wild-type FVIII. It would also be desirable to provide a functional recombinant FVIII protein with increased secretion as well as increased specific activity.
Previous studies have demonstrated that the B-domain of FVIII is dispensable for FVIII cofactor activity. Genetically engineered FVIII molecules that have varying degrees of B-domain deletion (BDD) yield secreted single chain FVIII species in which no intracellular proteolysis of the primary translation product is observed. These BDD FVIII mutants are advantageous because they are more efficiently produced in mammalian cells. Functional characterization of these BDD FVIII molecules demonstrated that FVIII cofactor activity is retained if thrombin cleavage after Arg372, Arg740 and Arg1689 occurs. Therefore, any functional construction of FVIII genetically engineered thus far generates a FVIIIa heterotrimer following thrombin activation. The functional advantages of previous BDD FVIII constructs has therefore been limited by rapid dissociation of the non-covalently linked A2 subunit from FVIIIa.
It would thus be desirable to provide improved recombinant FVIII protein. It would also be desirable to provide FVIIIa protein that is resistant to activation. It would further be desirable to provide FVIIIa protein that is APC-resistant. It would also be desirable to provide FVIII protein having increased secretion as compared to wild-type FVIII. It would further be desirable to provide FVIII protein having increased secretion and APC-resistance. It would also be desirable to provide FVIII protein having increased secretion and inactivation resistance. It would also be desirable to provide a method of treating hemophiliac patients with improved recombinant FVIII. It would further be desirable to provide a method for treating hemophiliac patients via replacement therapy, wherein the amount of FVIII protein required to treat the patient is decreased.